In a wireless communication system, passive intermodulation (PIM) is a form of intermodulation distortion that occurs in components of a base station normally thought of as linear, such as cables, connectors, and antennas. However, when subject to high radio frequency (RF) powers, as found in cellular systems, these devices can generate spurious signals. PIM appears as a set of unwanted signals created by the mixing of two or more strong RF signals in a non-linear device, such as in a loose or corroded connector, or in nearby rust. Other names for PIM include the diode effect and the rusty bolt effect.
FIG. 1 is a block diagram of a portion of a wireless communication system having a PIM source 4, and base stations 6a and 6b, referred to collectively herein as base stations 6. The base stations 6 are in communication with one or more wireless devices 8a and 8b, referred to collectively herein as wireless devices 8. The base stations 6 are also in communication with a backhaul network 10. The PIM source 4 may be located at or near a base station 6a. For example, the base station 6a may transmit a signal on the downlink which interacts with the PIM source 4 to produce PIM at a frequency that is within a band of receive signals received by the base station 6a, interfering with one or more desired receive signals.
Some frequencies at which PIM may occur can be predicted by the formulas, nF1−mF2, nF2−mF1, where F1 and F2 are carrier frequencies and the constants n and m are positive integers. When referring to PIM products, the sum of n+m is called the product order, so if m is 2 and n is 1, the result (2+1=3) is referred to as a third order product. Typically, the third order product is the strongest, causing the most harm, followed by the fifth and seventh order products, which can also cause significant harm. Because PIM amplitude becomes lower as the order increases, higher order products typically are not strong enough to cause direct frequency problems, but they usually assist in raising the adjacent noise floor. Note also that PIM may be generated by signals transmitted on a single transmit frequency F1 that is modulated in different ways and transmitted on two or more antennas, for example, when beamforming is implemented in the communication network.
Frequency planning for cellular systems is performed to make sure that the uplink and downlink frequency used at a given network node are separated adequately in frequency (for frequency division duplex (FDD) systems) and/or in time (for time division duplex (TDD) systems). This ensures that there exists a frequency separation between the downlink and uplink bands used at the same time and place. Without such planning, the high power transmit (TX) signal will interfere with the typically much lower power receive (RX) signal at the network node, e.g., base station. The result is referred to as self-interference. When a network node or adjacent nodes use multiple bands, the frequency planning should also consider not only the direct RX/TX frequency bands, but also possible PIM distortions occurring in the RX band that are generated by signals transmitted in the TX band, especially the third order PIM product. With the continually increasing need for higher bandwidth, operators are led to use multiple bands. For some band combinations, uplink transmissions on certain frequency sub-bands might start to be impacted by the intermodulation associated with the downlink transmissions over multiple sub-bands.
To reduce PIM, the network node site may be cleaned to prevent the rusty bolt effect, and the radio components such as cables may be chosen to minimize PIM. Even then, full control of the environment to avoid PIM is not always possible or practical.
Link adaptation is used to match modulation, coding and other signal and protocol parameters to the conditions on a radio link in an effort to maximize data throughput for a given set of radio link conditions. Coordinated link adaptation (CoLA) is a technique that includes sharing (or predicting) scheduling information among cells or network nodes using the same frequencies, such that some form of interference prediction for future transmissions presently scheduled is enabled as part of the link adaptation process. This prediction at scheduling time allows for a better tuning of the link adaptation and for higher throughput. Typically, the predicted instantaneous signal to interference plus noise ratio (SINR) estimate is used to select the most appropriate modulation and coding scheme (MCS). This is called the inner loop of the link adaptation.
With respect to link adaptation, in operation, if the SINR decreases, a lower order of modulation and coding may be selected and, conversely, if the SINR increases, a higher order of modulation and coding may be selected. Any bias in the SINR estimate of the inner loop is compensated by an outer loop of the link adaptation. This outer loop iteratively computes a SINR de-biasing parameter based on, for example, the acknowledgement or non-acknowledgement of correctly decoded transmitted cyclic redundancy check (CRC) data. Based on the SINR de-biasing parameter, the selected MCS may increase or decrease. An example of coordinated link adaptation, in the downlink (DL) of cellular wireless communication systems, is to share DL scheduling information between cells, keep the inner loop unchanged, but use multiple outer loops, which may be selected appropriately based on the knowledge of the scheduling activity (proper combination of active interfering cells) on the same frequency in the adjacent cells.